Hi,
You should get a little more control after some more experience. Also, if you make the PC muscles stronger that will help as well. To find that muscle, just stop the flow of urine while you are going pee. Work on contracting that muscle as often as possible. Trying not to think about it during helps as well, but I know that's difficult to do.
There are exercises which will help you last longer as well. Now there are devices which can do the exercises for you also. A really good solution is using the device in conjunction with the exercises for good results. That's what has helped me last longer with my wife.
Dont worry thats very normal for almost every guy. It wil get better as you go. Also, if you end up being with another girl at a later point in your life it will start all over again. It has to do with the excitement of doing something or someone new. The more you do it the longer you will last. One last tip, if your really bothered by it, try masturbating before you go out with her if you are anticipating sex, that will help a little.
There is no magical solution to such "problems". Being sensitive down there is quite natural, especially on younger people. You have just to find your way together in that.
Practice and frequent contact may help too. For example you can stop when you feel that the moment approaches, rest a little and then resume. Or you can keep yourself busy while she is doing it on you, for example do the same for her at the same time, although the outcome depends on how much you do enjoy giving oral pleasure to your girl (so the combination could "finish" you even earlier).
As I said, it is up to you to find your way. An understanding girl is very helpful in such situations.